DENVER — Frontier Airlines Holdings Inc. took one of its first significant steps to restructure operations Wednesday, ending a regional jet pact with Republic Airways and announcing plans to eliminate service to five cities.

The announcement, which was made by both airlines, comes as Denver-based Frontier reorganizes under a Chapter 11 petition in U.S. Bankruptcy Court.

It represents a shift away from a business strategy to move into small and midsized markets that were underserved by larger carriers. Frontier put it into place in January 2007 when it signed the 11-year agreement with Republic Airways Holdings Inc.

"Unfortunately, with current economic conditions and other business changes, we have been forced to drastically rethink the use of regional aircraft in our flight mix," Frontier Chief Executive Officer Sean Menke said in a statement.

Frontier will eliminate 12 Republic-owned 76-seat jets from daily operations by mid-June. Service will end by June 1 to Sioux City, Iowa, Jacksonville, Fla., Little Rock, Ark., Memphis, Tenn., and Tulsa, Okla. It will not start service, as planned, to Missoula, Mont.

Passengers who may be affected by the schedule changes will receive refunds or the option for an alternate flight, Frontier said.

Menke has said the airline was forced into bankruptcy reorganization because its credit card processing company sought to hold up to 100 percent of proceeds from ticket sales in reserve until the passengers' flights were completed.